Nasa plans to capture an asteroid and send astronauts to study it, the US space agency announced in a statement outlining its new budget on Wednesday.

The effort is intended to advance the knowledge of asteroids, and how to defend Earth from asteroid crashes.

"We are developing a first-ever mission to identify, capture and relocate an asteroid," Nasa administrator Charles Bolden said. "This mission represents an unprecedented technological feat that will lead to new scientific discoveries and technological capabilities and help protect our home planet."

Asteroids are minor planets that orbit the sun. Most of the known ones hang together in the asteroid belt between Mars and Jupiter. Nasa would try to pick off a close one.

The agency released an animated video depicting how a space rock measuring about 9 meters wide would be captured and corralled for study. (Some space watchers say an asteroid under 10 meters wide is technically a meteoroid.)

An unmanned spacecraft with a giant telescoping plunger would fly to the asteroid, suck it in, and secure it in a truly industrial-strength Hefty bag of sorts. Then the craft would fly the asteroid into a steady orbit, possibly around the moon.

Then astronauts would fly up and hook onto it and break pieces off for research. Learning what asteroids are made of "could help us understand the origins of the solar system and inform decisions about how to conduct missions to distant planetary bodies," Nasa says.

The mission could be carried off as soon as the early 2020s. Nasa already has simulated an asteroid mission to figure out how astronauts might undertake a spacewalk on an asteroid. "Decisions made about ways to best sample an asteroid will be informed by the agency's ongoing concept development and past work," the space agency says.

The provisional Nasa budget for the 2014 fiscal year is $17.7bn, down $50m from 2012.